Referring to FIG. 1, a fuel supply system for a diesel vehicle includes a fuel filter 20 for filtering a fuel in a fuel tank 10 to supply the filtered fuel to an engine. A thermo-switch 22 for detecting a temperature of the fuel, a heater 24 that is switched on and off according to the detected temperature of the fuel, a moisture sensor 26 for detecting whether the amount of moisture filtered from the fuel is larger than a predetermined amount to switch on an alarm lamp of a cluster 30, and the like are mounted on the fuel filter 20.
The heater 24 of the fuel filter 20 improves low temperature startup performance, and lowers viscosity of the fuel by using the heater to smoothly supply the fuel in order to solve a problem in which viscosities of some components (for example, paraffin) of the fuel increase and the fluidity of the fuel decreases, when the temperature of the diesel fuel decreases to be lower than a reference low temperature value, due to the characteristics of the diesel fuel.
Here, a configuration and an operation of a heater circuit mounted on a diesel fuel filter according to the related art will be described below.
FIG. 2 is a circuit diagram of a heater circuit mounted on a diesel fuel filter according to the related art.
In FIG. 2, the heater 24 is mounted within a fuel filter 20, and the heater is a positive temperature coefficient (PTC) heater.
A thermo-switch 22 is mounted within the fuel filter 20 as a bimetal switch type for detecting the temperature of the fuel from the fuel tank.
Then, the heater 24 is connected to the battery through a relay 28 for a heater power source, and the thermo-switch 22 switches on and off the relay 28 for a heater power source in an ignition step IG2.
Accordingly, if the thermo-switch 22 detects the temperature of the fuel supplied from the fuel tank to the fuel filter 20 and it is determined that the temperature of the fuel is in a low temperature range or less, the thermo-switch 22 is switched on and the relay 28 for a heater power source is also switched on at the same time, so that power of a battery is supplied to the heater 24 to heat the fuel according to the operation of the heater 24.
When the temperature of the fuel supplied from the fuel tank to the fuel filter 20 is higher than the low temperature range, the thermo-switch 22 is maintained in an off state and the relay 28 for a heater power source is also maintained in the off state, so that the heater is not operated and the fuel is not heated.
Then, the fuel temperature detection range for switching on and off the thermo-switch 22 is set to be, for example, in a case of a heater/thermo-switch separable type, a temperature of the fuel −3° C.±3 in a case of an on operation and a temperature of the fuel +5° C.±3 in a case of an off operation, and in a case of a heater/thermo-switch integral type, a temperature of the fuel −3° C.±3 in a case of an on operation and a temperature of the fuel +17° C.±3 in the case of an off operation.
However, because a deviation of a fuel temperature detection range measured by the thermo-switch, that is, a difference between a fuel temperature detection range for an on operation of the thermo-switch and a fuel temperature detection range for an off operation of the thermo-switch is larger, the temperature of the fuel cannot be precisely detected, and thus, the on/off control of the heater cannot be precisely performed.
Furthermore, because the fuel temperature detection range detected by the thermo-switch is large, an on/off operation of the thermo-switch is frequently performed, so that the durability of the thermo-switch deteriorates and may easily break down.
In addition, a connector for a thermo-switch including a connector for a heater connected to a relay for a heater power source is consumed, manufacturing costs increase, and the number of processes for mounting the connector in an inline assembly process also increases.